1. Field of the Invention
The present invention relates to a DNA sequence encoding hyaluronan synthase from Pasteurella multocida. More particularly, the present invention relates to a DNA sequence encoding hyaluronan synthase from Pasteurella multocida which is capable of being placed into a recombinant construct so as to be able to express hyaluronan synthase in a foreign host. The present invention also relates to methods of using a DNA sequence encoding hyaluronan synthase from Pasteurella multocida to (1) make hyaluronan polymers of varying size distribution; (2) make hyaluronan polymers incorporating substitute or additional base sugars; (3) develop new and novel animal vaccines; and (4) develop new and novel diagnostic tests for the detection and identification of animal pathogens.
2. Brief Description of the Background Art
The polysaccharide hyaluronic acid (“HA”) or hyaluronan is an essential component of higher animals that serves both structural and recognition roles. In mammals and birds, HA is present in large quantities in the skin, the joint synovial fluid, and the vitreous humor of the eye. Certain pathogenic bacteria, namely, Gram-positive Group A and C Streptococcus and Gram-negative Pasteurella multocida Carter Type A, produce extracellular capsules containing HA with the same chemical structure as the HA molecule found in their vertebrate hosts. This “molecular mimicry” foils attempts to mount a strong antibody response to the capsular polysaccharide. In contrast, capsular polysaccharides with different structures produced by other bacteria are often quite antigenic. The HA capsule also apparently helps the pathogens evade host defenses including phagocytosis.
Historically, researchers in the field have not succeeded in cloning or identifying Hyaluronan Synthase (“HAS”) from Pasteurella. Bacterial HAS enzymes from Group A & C Streptococcus have been identified and cloned. HasA from Streptococcus pyogenes was the first HAS to be definitively identified. This integral membrane protein utilizes intracellular UDP-GlcA and UDP-GlcNAc as substrates. The nascent HA chain is extruded through the membrane to form the extracellular capsule. A Xenopus protein, DG42, has also been determined to be a HAS. Several human and murine homologs of DG42, named HAS1, HAS2 and HAS3, have also been identified. There is considerable similarity among these molecularly cloned mammalian enzymes at the amino acid level, but they reside on different chromosomes. The unique HAS from P. multocida has a primary structure that does not strongly resemble the previously cloned enzymes from Streptococcus, PBCV-1 virus or higher animals.
A viral HAS, with an ORF called A98R, has been identified as being 28-30% identical to the streptococcal and vertebrate enzymes. PBCV-1 (Paramecium bursaria Chlorella virus) produces an authentic HA polysaccharide shortly after infection of its Chlorella-like green algae host. A98R is the first virally encoded enzyme identified as producing a carbohydrate polymer.
Carter type A P. multocida, the causative agent of fowl cholera, is responsible for great economic losses in the U.S. poultry industry. Acapsular mutants of P. multocida do not thrive in the bloodstream of turkeys after intravenous injection, where encapsulated parental strains multiply quickly and cause death within 1 to 2 days. Spontaneously arising mutant strain which is acapsular, was also 105-fold less virulent than wild-type, but the nature of the genetic defects in all the cases before the disclosed mutant (as described hereinafter) was not known.
Pasteurella bacterial pathogens cause extensive losses to U.S. agriculture. The extracellular polysaccharide capsule of P. multocida has been proposed to be a major virulence factor. The Type A capsule is composed of a polysaccharide, namely HA, that is identical to the normal polysaccharide in the host's body and thus invisible to the immune system. This “molecular mimicry” also hinders host defenses such as phagocytosis and complement-mediated lysis. Furthermore, HA is not strongly immunogenic since the polymer is a normal component of the host body. The capsules of other bacteria that are composed of different polysaccharides, however, are usually major targets of the immune response. The antibodies generated against capsular polymers are often responsible for clearance of microorganisms and long-term immunity.
Knowing the factors responsible for a pathogen's virulence provides clues on how to defeat the disease intelligently and efficaciously. In Type A P. multocida, one of the virulence factors is the protective shield of nonimmunogenic HA, an almost insurmountable barrier for host defenses. A few strains do not appear to rely on the HA capsule for protection, but utilize other unknown factors to resist the host mechanisms. Alternatively, these strains may possess much smaller capsules that are not detected by classical tests.
For chickens and especially turkeys, fowl cholera can be devastating. A few to 1,000 cells of some encapsulated strains can kill a turkey in 24-48 hours. Fowl cholera is an economically important disease in North America. Studies done in the late 1980s show some of the effects of fowl cholera on the turkey industry: (i) fowl cholera causes 14.7 to 18% of all sickness, (ii) in one state alone the annual loss was $600,000, (iii) it costs $0.40/bird to treat a sick flock with antibiotics, and (iv) it costs 0.12/bird for treatment to prevent infection.
Certain strains of Type A P. multocida cause pneumonic lesions and shipping fever in cattle subjected to stress. The subsequent reduction in weight gain at the feedlot causes major losses. The bovine strains are somewhat distinct from fowl cholera strains, but the molecular basis for these differences in host range preference is not yet clear. Type A also causes half of the pneumonia in swine. Type D P. multocida is most well known for its involvement in atrophic rhinitis, a high priority disease in swine.
Type D capsular polymer has an unknown structure that appears to be some type of glycosaminoglycan; this is the same family of polymers that includes HA. This disease is also precipitated by Bordetella bronchiseptica, but the condition is worse when both bacterial species are present. It is estimated that Type F causes about 10% of the fowl cholera cases. In this case, the capsular polymer is not HA, but a related polymer called chondroitin.
Currently, disease prevention on the fowl range is mediated by two elements: vaccines and antibiotics, as well as strict sanitation. The utility of the first option is limited, since there are many serotypes in the field and vaccines are only effective against a limited subset of the entire pathogen spectrum. Killed-cell vaccine is dispensed by labor-intensive injection, and the protection obtained is not high. Therefore, this route is usually reserved for the breeder animals. More effective live-cell vaccines can be delivered via the water supply, but it is difficult to dose a flock of thousands evenly. Additionally, live “avirulent” vaccines can sometimes cause disease themselves if the birds are otherwise stressed or sick. The most common reason for this unpredictability is that these avirulent strains arose from spontaneous mutations in unknown or uncharacterized genes. Protocols that utilize repeated alternating exposure to live and dead vaccines can protect birds only against challenge with the same serotype.
The second disease prevention option is antibiotics. These are used at either subtherapeutic doses to prevent infection or at high doses to combat fowl cholera in infected birds. The percentage of birds with disease may drop with drug treatment, but timely and extensive treatment is necessary. Late doses or premature withdrawal of antibiotics often results in chronic fowl cholera and sickly birds with abscesses or lesions that lead to condemnation and lost sales. Furthermore, since resistant strains of P. multocida continually arise and drug costs are high, this solution is not attractive in the long run. In addition, Type F P. multocida may cause 5-10% of fowl cholera in North America. A vaccine directed against Type A strains may not fully protect against this other capsular type if it emerges as a major pathogen in the future. In the cattle and swine industries, no vaccine has been totally satisfactory. Prophylactic antibiotic treatment is used to avoid losses in weight gain, but this option is expensive and subject to the microbial resistance issue.
In the present invention, enzymes involved in making the protective bacterial HA capsule have been identified at the gene/DNA level. The identification of these enzymes will lead to disease intervention by blocking capsule synthesis of pathogens with specific inhibitors that spare host HA biosynthesis. For example, a drug mimicking the substrates used to make HA or a regulator of the P. multocida HA synthase stops production of the bacterial HA polysaccharide, and thus blocks capsule formation. This is a direct analogy to many current antibiotics that have dissimilar effects on microbial and host systems. This approach is preferred because the P. multocida HA synthase and the vertebrate HA synthase are very different at the protein level. Therefore, it is likely that the enzymes also differ in reaction mechanism or substrate binding sites.
P. multocida, once stripped of its protective capsule shield is significantly more vulnerable a target for host defenses. Phagocytes readily engulfed and destroyed by the acapsular microbes. The host complement complex reaches and disrupts the sensitive outer membrane of bacteria. Antibodies are more readily generated against the newly exposed immunogens, such as the lipopolysaccharides and surface proteins that determine somatic serotype in P. multocida. These antibodies are better able to bind to acapsular cells later in the immune response. Thus, the immune response from vaccinations are more effective and more cost-effective. Capsule-inhibiting drugs are substantial additions to the treatment of fowl cholera.
The present invention and use of the capsule biosynthesis of Type A P. multocida aids in the understanding of the other capsular serotypes. DNA probes have been used to type A capsule genes to establish that Type D and F possess similar homologs.
High molecular weight HA also has a wide variety of useful applications—ranging from cosmetics to eye surgery. Due to its potential for high viscosity and its high biocompatibility, HA finds particular application in eye surgery as a replacement for vitreous fluid. HA has also been used to treat racehorses for traumatic arthritis by intra-articular injections of HA, in shaving cream as a lubricant, and in a variety of cosmetic products due to its physiochemical properties of high viscosity and its ability to retain moisture for long periods of time. In fact, in August 1997 the U.S. Food and Drug Agency approved the use of high molecular weight HA in the treatment of severe arthritis through the injection of such high molecular weight HA directly into the affected joints. In general, the higher molecular weight HA that is employed the better. This is because HA solution viscosity increases with the average molecular weight of the individual HA polymer molecules in the solution. Unfortunately, very high molecular weight HA, such as that ranging up to 107, has been difficult to obtain by currently available isolation procedures.
To address these or other difficulties, there is a need for new methods and constructs that can be used to produce HA having one or more improved properties such as greater purity or ease of preparation. In particular, there is a need to develop methodology for the production of larger amounts of relatively high molecular weight and relatively pure HA than is currently commercially available. There is yet another need to be able to develop methodology for the production of HA having a modified size distribution (HAΔsize) as well as HA having a modified structure (HAΔmod).
The present invention, therefore, functionally characterizes the Type A P. multocida genes involved in capsule biosynthesis, assesses the role of the capsule as a virulence factor in fowl cholera, and has obtained the homologous genes involved in Type D and F capsule biosynthesis. With this information, vaccines have been developed utilizing “knock out” P. multocida genes that do not produce HAS. These acapsular avirulent strains have the ability to act as vaccines for fowl cholera or shipping fever.